Molecular level coating of metal oxide particles

ABSTRACT

Polymer encapsulated metal oxide particles are prepared by combining a polyamide acid in a polar aprotic solvent with a metal alkoxide solution. The polymer was imidized and the metal oxide formed simultaneously in a refluxing organic solvent. The resulting polymer-metal oxide is an intimately mixed commingled blend, possessing synergistic properties of both the polymer and preceramic metal oxide. The encapsulated metal oxide particles have multiple uses including, being useful in the production of skin lubricating creams, weather resistant paints, as a filler for paper, making ultraviolet light stable filled printing ink, being extruded into fibers or ribbons, and coatings for fibers used in the production of composite structural panels.

This application is a divisional patent application of, commonly ownedpatent application Ser. No. 08/742,068, filed Oct. 31, 1996 and now U.S.Pat. No. 6,114,156.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and a National Research Council Associate which may beused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

FIELD OF THE INVENTION

This invention relates generally to the preparation of coatings, moldingpowders, fibers, films and matrix resins for composites. The inventionrelates specifically to the molecular level coating of metal oxideparticles with polyimides and the product(s) obtained thereby.

BACKGROUND OF THE INVENTION

The development of advanced high performance polymers for aerospaceapplications has been, and remains, a particularly active area ofresearch. High performance polyimides have found extensive use in theaerospace industry as adhesives, and more recently as matrix resins forcomposites, molding powders and films.

Improvements in high performance systems are motivated by the search foradvanced materials with improved or unique properties. Previous researchinto polyimide modifications have involved the simplest and mostinexpensive methods involving manipulation of the chemical compositionof mainly linear polyimides. An alternative method of modification is byincorporation of a second component of differing chemical structure orcomposition.

Modifications utilizing the morphology of multiphase systems with phasesof differing chemical structure allows for a remarkable balance ofdiverse properties. This is especially true when at least one phase ison the molecular scale, allowing for a balance of diverse properties.The production of organic-inorganic hybrid materials may take placethrough several different methods. One route is by direct mixing of lowmelt glasses with engineering thermoplastics, Beall and Quinn (Phosphateglass-polymer emulsions”, Ceramic Transactions, Volume 33, 1993).Organic-inorganic blends have also been formed by intercalation ofpolymers in the melt between mica sheets; Giannelis (“A New Strategy ForSynthesizing Polymer-Ceramic Nanocomposites”, Journal of the Minerals,Metals and Materials Society, Volume 44. Number 3, 1992). Theseprocesses have resulted in a class of materials called ceramers thatpossess properties of both inorganic glasses and organic polymers.

Organic-inorganic blends may also be produced by utilizing the sol-gelprocess. With this method, the inorganic phase is formed in-situ byhydrolysis and polycondensation of the alkylated metal aldoxides.Alkylated metal oxides are organic low molecular weight compoundssoluble in organic solvents which precipitate as metal oxides uponcondensation. Sol-gel ceramers in the past have involved the formationof transparent or translucent thin films where the organic and inorganicphases are co-mingled and then cured, as described by lyoku et al, (ThePreparation of New Poly(phenylsilsesquioxane)-Polyimide Hydrid Films bythe Sol-Gel Process and Their Properties”, High Performance Polymers,Volume 6, 1994), where they indicate the formation of small particles ofsilicone dispersed in a film.

In another case where inorganic-organic composites are formed, thefunctionality of the poly(dimethylsiloxane) chains of the polymerresults in strong interactions between the two components, where thepolymer constitutes the continuous phase, while the ceramic materialserves as reinforcing particles. When the polymer is present in lowerconcentrations, it becomes dispersed in the continuous ceramic phase.Mark et al, (“Inorganic-Organic Composites Including Some ExamplesInvolving Polyamides and Polyimides”, Macromolecular Symposium, Volume98, 1995), even cites cases where a bicontinuous system is formed.

Another method of generating organic-inorganic blended materials is byencapsulation. This technology is being used extensively in manyindustries and for a wide variety of materials. Microcapsules can havemany different structures, but typically involve a core regionsurrounded by a shell. The geometry may be spherical or irregular, andcontain a continuous core or small particles of core material surroundedby the shell. As a result of agglomeration, traditional methods ofencapsulating metal oxide particles result in amulti-molecular/multi-nuclear core region surrounded by a coating. MacroCoated Particle (MCP) technology results in organic-inorganic particlesin the ten to hundreds of micron range (FIG. 1). Molecular Level Coating(MLC) technology, as employed in the present invention (FIG. 2),utilizes microencapsulation technology in conjunction with sol-gelprocessing. The in-situ generation of the inorganic phase with MLCresults in a polymer coated, molecular level, metal oxide particle inthe angstrom size range.

Preparation of a ceramer by MLC results in the formation of the metaloxide as a discrete particle thinly coated with a polymer. MLC of apreceramic and a high performance polymer facilitates the design ofsystems that combine the thermal stability, high stiffness (modulus) orlight reflective properties of a glass with the toughness andprocessability of a polymer. MLC further offers the advantage of metaloxide particles with less abrasive properties than uncoated metaloxides.

Titanium oxide, a commonly used whitener in pigments and coatings, issubject to weathering with long term exposure to sunlight. Exposure toultraviolet light results in excitation of the electrons in the titaniumcompound which may return to the ground state by transferring freeradicals to the surrounding materials. Absorption of these free radicalsby the surrounding organic material leads to discoloration anddegradation. In accordance with the present invention, degradation oftitanium oxide is slowed by encapsulating the titanium oxide particlesin a polymer that is nonreactive to free radical bombardment.

SUMMARY OF THE INVENTION

It an object of the present invention to provide a molecular levelcoated metal oxide particle that has less abrasive properties thanuncoated metal oxide particles.

A further object of the present invention is to provide a metal oxideencapsulated with a polyimide that has synergistic propertycharacteristics.

Another object of the present invention is to provide encapsulatedtitanium oxide particles to decrease the degradation and improve theweathering and colorfast property characteristics of the particles.

An additional object of the present invention is a process ofencapsulation of metal oxide particles with a surrounding insulation ofa polyimide to thereby insulate the metal oxide from free radicaltransfer and provide better weathering and good resistant color fastproperties to the metal oxide.

A further object of the present invention is to provide an encapsulatedtitanium oxide for use in a protective urguent for human skin.

An additional object of the present invention is to provide coated metaloxide particles that hinder the loss of free radicals when exposed toultraviolet light.

Another object of the present invention is a process of preparingmolecular level coatings of metal oxide particles for use as ultravioletand weather protectives in unguents for human skin, paper fillers,printing inks, fiber reinforced composites and textiles.

Another object of the present invention is a polymer encapsulated metaloxide matrix resin for manufacturing fiber reinforced composites whereinthe metal oxide particles increase the modulus of the polymer in thecomposite.

The foregoing and additional objects are attained by employing, as themetal oxide coating, a polyimide having repeating units of:

wherein Ar is a member selected from the group consisting of:

wherein the catenation is meta, meta; meta, para; or para, para;

wherein R is a member selected from the group consisting of:

and, wherein n is an integer in the range of 10 to 10,000.

Polymer encapsulated metal oxide particles were prepared by combining apolyamide acid in a polar aprotic solvent with a metal alkoxidesolution. The polymer was imidized and the metal oxide formedsimultaneously in refluxing organic solvent. The resultant polymer-metaloxide is an intimately mixed commingled blend, possessing properties ofboth the polymer and preceramic metal oxide.

Polymers suitable for practice of the present invention are disclosed inthe following U.S. Patents (incorporated herein by reference), U.S. Pat.No. 4,094,482 (LARC™ TPI); U.S. Pat. No. 4,603,061 (LARC™ 6F); U.S. Pat.No. 4,937,317 (LARC™ ITPI);, and U.S. Pat. No. 5,147,966 (LARC™ IA), andare commercially available from NASA licensees of these patents.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be more readily apparent as the same becomesbetter understood in connection with the following drawings and specificexamples wherein:

FIG. 1 is a schematic representation of a polymer coating of macro metaloxide particles;

FIG. 2 is a schematic representation of a polymer coating of a molecularlevel metal oxide particle;

FIG. 3 is a bar graph showing the improved flexural modulus of thepolymer coated molecular level metal oxide particles over the unmodifiedpolymer;

FIG. 4 is a bar graph showing the improved flexural strength of thepolymer coated molecular level metal oxide particles over that of theunmodified polymer;

FIG. 5 is a bar graph showing the flexural strength of a compositespecimen formed of metal oxide-polyimide prepared according to thepresent invention employed as a carbon fiber coating and molded into acomposite panel; and,

FIG. 6 is a bar graph showing the flexural modulus of the compositespecimen formed of metal oxide-polyimide prepared according to thepresent invention employed as a carbon fiber coating and molded into acomposite panel.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 is a schematic representation of amacro coated particle, designated generally by reference numeral 10 isshown. Coated particle 10 consists of a multimolecular metal oxideparticle 12 having a polymer coating 14 thereon. These coated particlesare in the micron size range and are developed by Macro Coated ParticleTechnology (MCP).

FIG. 2 is a schematic representation of a molecular metal oxide coatedparticle, generally designated by reference numeral 20. Coated particle20 consists of a single molecule of a metal oxide 22 having a polymercoating 24 thereon. This coated particle is in the angstrom size rangeand is developed by Molecular Level Coating Technology (MLC). Thepresent invention is confined to Molecular Level Coating Technology(MLC) and the products produced thereby.

These encapsulated particles offer properties of the metal oxide andpolymer which are not related simply to a rule of mixtures but appear tobe synergistic in character. Encapsulation results in insulation of thesurrounding medium from free radical transfer resulting in materialswith better weathering and good resistant color fast systems.

The insulative coating serves as protection from the environment. Thus,in one application of the present invention, the metal oxide, such asTiO₂, is mixed with a cosmetic base to afford a resulting materialhaving enhanced stability to ultraviolet light, to serve as a protectiveunguent for human skin. The photosensitive TiO₂ is isolated from thecosmetic binder thereby decreasing binder decomposition as a result offree radical transfer. This insulating effect also serves to protect theskin from negative physiological effects due to free radical attack thatoften results in adverse chemical reactions.

When employing the coated metal oxide particles of the present inventionas a filler for paints and coatings, the insulative properties of thecoated metal oxide particles are effective. The supporting medium of thecoating and its surrounding environment, is protected by theseinsulative properties from free radical transfer as a result ofultraviolet exposure to thereby decrease chalking.

When employing the coated metal oxide particles of the present inventionas a filler for papers, the encapsulated metal oxide serves as awhitener to the paper, while decreasing the yellowing normally caused byenvironmental exposure.

The coated metal oxide particles of the present invention are alsouseful as an additive to printing inks. In this environment, the highperformance polymer provides lubrication to the pigments, resulting in aless chalky medium with decreased friction, while the metal oxideprovides the desired pigmentation.

As will be further described hereinafter, when employing the polymercoated metal oxide particles of the present invention as a matrix resinfor fiber reinforced composites, the metal oxide particles increase theflexural modulus of the polymer in the composite.

When employing the polymer coated metal oxide particles of the presentinvention as a filler for textiles, the metal oxide serves as a whitenerand the insulative properties of the high performance polymer serves toprotect the fiber from weathering.

DESCRIPTION OF THE PREFERRED EMBODIMENTS AND SPECIFIC EXAMPLES

In preparing the polyimide-titanium oxide blend, the polyamide acid formof LARC™ IA in N-methyl pyrrolidinone (NMP) and available from ImitechCorporation, was imidized by introducing it directly into a mixture ofrefluxing o-zylene and alkyl titanate, (both available from AldrichChemical Company). The water formed from ring closure during imidizationfacilitated the formation of the metal oxide. This results in theformation of the metal oxide as a discrete particle thinly coated withthe polymer. The fine powder was recovered and dried under vacuum 12hours at 200° C. All of the systems formed fine powders which dispersedwell in lacquers, oil based coatings, and epoxy. In lieu of N-Methylpyrrolidinone, gamma-butyrolactone, N,N-dimethylacetamide,1,3-dimethyl-2-imidazolidinone, and others, may be utilized as thesolvent in this process. Although the polyimides employed in thespecific examples herein were from the soluble polyamide acid forms,soluble polyimides may also be used, in particular, LARC™ IA and LARC™6F.

Example I

Reaction Sequence for the Synthesis of a Polymer-metal OxideEncapsulated Molecular Level Particle

Into a resin kettle equipped with a mechanical stirrer, nitrogen inlet,moisture trap and reflux condenser was added 300 ml of o-xylene. Theresin kettle was wrapped in glass wool and the solvent heated to reflux.

A 10 g sample of 10 weight percent solution of polymer in NMP wascombined with 10 g of metal alkoxide and 10 g of water and allowed tostir until homogeneous. The resin mixture was added dropwise via anaddition funnel into the refluxing o-xylene. The system was allowed toheat at approximately 140° C., with stirring overnight, the solidscollected and dried under vacuum 12 hours at 200° C. Yield 11 grams offine off-white powder. Examples of polymer oxides formed are summarizedin Table I.

TABLE I Titanium Zirconium Silica Polymer Oxide Oxide Oxide LARC ™ IAOff-white Light Light powder orange brown powder powder LARC ™ TPIOff-white Light Light powder orange brown powder powder LARC ™ I-TPIOff-white Light Light powder orange brown powder powder LARC ™ 6-F WhiteLight Off-white powder pink powder powder

Example II

The same as in Example I except gamma-butyrolactone was employed as thesolvent in lieu of NMP.

Example III

The same as in Example I except, in lieu of NMP, the solvent employed isN,N-dimethylacetamide.

Example IV

The same as in Examples I except, in lieu of NMP, the solvent employedis 1,3-dimethyl-2-imidazolidinone.

Example V

Synthesis of a Polymer-titanium Oxide Blend

Into a 10 liter resin kettle equipped with a mechanical stirrer,nitrogen inlet, moisture trap and reflux condenser was placed 5,000 mlof o-xylene. The resin kettle was wrapped in glass wool and heated toreflux.

1330 ml of tetrakis(2-ethylhexyl)orthotitanate (alkyl titanate) wasadded to the hot o-xylene and allowed to heat 30 minutes atapproximately 140° C.

LARC™ IA, 230 g of 30 weight percent in NMP at 3% stoichiometric offsetendcapped with phthalic anhydride was diluted with 1300 g of distilledNMP. The resin mixture was added dropwise via an addition funnel intothe refluxing o-xylene over a 2 hour period. The system was allowed toheat at approximately 140° C. with stirring for 48 hours. The lightbrown slurry was centrifuged and the solvent decanted off. The recoveredoff-white powder was washed in o-xylene, collected on medium porositysintered glass, and dried 12 hours at 200° C. under vacuum. The yieldwas 63.7 g, 5% weight loss by thermogravimetric analysis at 324° C.

Example VI

The same as Example V except, in lieu of NMP, the solvent employed isgamma-butyrolactone.

Example VII

The same as Example V except, in lieu of NMP, the solvent employed isN,N-dimethylacetamide.

Example VIII

The same as Example V except, in lieu of NMP, the solvent employed is1,3-dimethyl-2-imidazolidinone.

The molar ratios of metal oxide to polymer formed in Examples V to VIIIare summarized in TABLE II below:

TABLE II Alkyl Molar Ratio of LARC ™ IA Titanate H₂O Metal Oxide MolesMoles Moles to Polymer 3.2 × 10⁻² 8.6 × 10⁻² 2.68 2.7:1 9.5 × 10⁻²  .515.4:1 0.15 2.3  15:1

Example IX

Preparation of a Skin Unguent

A skin unguent was prepared by mixing one part, by weight, of the powderprepared in Example I to 1 to 10 parts, by weight, of mineral oil.

Example X

A skin unguent was prepared by mixing one part, by weight, of the powderprepared in Example I, with 1 to 10 parts by weight, of glycerin.

Example XI

A skin unguent was prepared by mixing one part, by weight, of the powderprepared in Example I, with an emulsion comprising 0.5 to 4 parts, byweight, of glycerin; 0.5 to 4 parts, by weight, of mineral oil; and 0.25to 1 part, by weight, of water.

In each of Examples IX, X and XI, a resulting unguent having a colorindicative of that of the polymer powder employed, was obtained that,when spread on the skin, left a protective coating.

The ratios of the ingredients in the skin unguent produced by ExamplesIX, X and XI are summarized in TABLE III below:

TABLE III MLC Powder Mineral Oil Glycerin Water (mg) (mg) (mg) (mg) 250250 250 1250  250 2500  250 250 250 1250  250 2500  250 125 125  10 250250 250  25 250 500 500 100 250 1000  1000  250

Example XII

Preparation of Filled Paints and Coatings

A 50 mg aliquot of the sample prepared in Example I was combined with100 mg of binder. The binder was allowed to dry 24 hours and a light tancoating resulted. Table IV summarizes the binders employed in thepreparation of the paints and coatings.

TABLE IV mg of encapsulated Binder (100 mg) TiO₂ Result Tung Oil 50Light Tan Coating/Paint Clear Vinyl Lacquer 50 Light Tan Coating/PaintEpoxy Resin 50 Light Tan Cured Epoxy Rubber Cement 50 Light Tan FilmFlexible

The lacquer employed in the Example above was “SO SURE” lacquer,obtained from LHB Industries, Berkley, Mo.; the epoxy resin was “bisphenol A diglycidyl ether” with polymercapton hardener, and acquiredfrom the Devcon Corporation; and the rubber cement was “Carter's RubberCement” (Carter's Ink Division) and acquired from Demmison ManufacturingCompany.

Example XIII

Preparation of a Filled Paper

8 g of cellulose pulp was slurried with 1 g of the powder prepared inExample II. The water was extracted and the filled pulp was collectedover a suction apparatus at approximately 15 psi and allowed to dry atambient temperature.

Example XIV

A 5 g aliquot of the powder prepared in Example II was added to 50 g ofprinting ink (“Numbering Ink”, acquired from Bates ManufacturingCompany). The filled ink was used to print on paper and exhibitedstability to UV light. Ratios, other than the 1:10 polymer to ink,employed in this specific example would be expected to also be operativeto provide a UV light stable ink.

Example XV

Reaction Sequence for Preparation of a Polymer-silica Oxide Blend

Into a 20 liter resin kettle equipped with a mechanical stirrer,nitrogen inlet, moisture trap and reflux condenser, was placed 12,000 mlof o-xylene. The resin kettle was wrapped in glass wool and heated toreflux.

LARC™ IA (1455 g, 30 weight percent) in NMP, at 3% stoichiometric offsetand endcapped with phthalic anhydride, was diluted to 15% solids withdistilled NMP. An alkyl silicate, tetraethylorthosilicate (TEOS, 436 ml)and distilled water (436 ml) was added slowly to the resin mixture. Thesolution was stirred for 5 hours, then added dropwise via an additionfunnel into the resin kettle of refluxing o-xylene over a 2 hour period.The system was allowed to heat at approximately 140° C. with stirring,for 16 hours. During the duration of heating, 780 ml of aqueous materialwas collected in the moisture trap. The light brown slurry wascentrifuged and the solvent decanted off. The remaining light brownpowder was collected over medium porosity sintered glass and dried 12hours at 200° C. under vacuum. Test specimens yielded improvedmechanical properties over the unmodified polymer and are presented inFIGS. 3 and 4. Examples of molar ratios of metal oxide to polymer aresummarized in TABLE V below:

TABLE V Molar Ratio Alkyl Alkyl of Metal LARC ™ IA Silicate ZirconateH₂O Oxide to Moles Moles Moles Moles Polymer 0.92 5.4  48 6:1 0.26 0.786.9 3:1 5.3 × 10⁻² 0.11 2:1 5.3 × 10⁻² 1.5 × 10⁻² 6.9 × 10⁻² 0.141.5:1   5.3 × 10⁻² 7.8 × 10⁻² 3.8 × 10⁻² 0.69 2:1

Example XVI

The powder obtained from Example XV was passed through a Brabender meltextruder heated to 315° C. at a volume rate of 0.0105 cm³ sec⁻¹. Meltextrusion yielded polymer fiber or ribbon.

Example XVII

Fiber Reinforced Composite Panel

A powder coated towpreg was prepared by coating carbon fibers with themodified polymer prepared in Example XV. The powder coated towpreg wasthen wound around a frame, stacked in a mold and consolidated under 300psi one hour at 350° C. The composite specimen was slowly cooled to roomtemperature prior to removal from the mold. Test panels yielded panelswith the strength properties plotted in FIG. 5 and modulus properties asplotted in FIG. 6.

Example XVIII

Reaction Sequence for the Synthesis of a Polymer-metal Oxide/Metal OxideEncapsulated Particle

Into a resin kettle equipped with a mechanical stirrer, nitrogen inlet,moisture trap and reflux condenser was placed 300 ml of o-xylene. Theresin kettle was wrapped in glass wool and the solvent heated to reflux.

A 10 g sample of 10 weight percent solution of polymer in NMP wascombined with 1 g of alkyl silicate (TEOS) and 9 g of zirconium butoxideand allowed to stir until homogeneous. The resin mixture was addeddropwise via an addition funnel into the refluxing o-xylene. The systemwas allowed to heat at approximately 140° C. with stirring overnight,the solids collected and dried under vacuum 12 hours at 200 ° C.

Example XIX

Synthesis of a Polymer-metal Oxide/Metal Oxide Encapsulated Particle

Into a resin kettle equipped with a mechanical stirrer, nitrogen inlet,moisture trap and reflux condenser, was placed 300 ml of o-xylene. Theresin kettle was wrapped in glass wool and the solvent heated to reflux.

A 10 g sample of 10 weight percent solution of polymer in NMP wascombined with 9 g of alkyl silicate (TEOS) and 1 g of zirconium butoxideand allowed to stir until homogeneous. The resin mixture was addeddropwise via an addition funnel into the refluxing o-xylene. The systemwas allowed to heat with stirring overnight, the solids collected anddried under vacuum 12 hours at 200° C.

The foregoing specific examples are given to illustrate the principal ofthe invention and, as such, are to be considered as exemplary and notexhaustive. There are numerous modifications and variations of thepresent invention that will be readily apparent to those skilled in theart in the light of the above teachings.

For example, where specific quantities and ratios are employed it is tobe understood that the invention is not so limited and that thesespecifics are to illustrate specific examples and reactions, and are notto serve as limitations on the invention. Other quantities and ratiosthat may be apparent to those skilled in the art, and within the scopeof the appended claims, are intended to be included herein.

It is therefore to be understood that, within the scope of the appendedclaims, the invention may be practiced other than as specificallydescribed herein.

What is claimed as new and desired to secured by Letters Patent of theUnited States is:
 1. A process of making a polymer encapsulated metaloxide product wherein the individual molecules of the metal oxide arecoated with the polymer comprising the steps of: combining a polyamideacid in a polar aprotic solvent with a metal alkoxide solution,refluxing the resulting solution to simultaneously imidize the polymerand form the metal oxide to provide an intimately mixed commingledblend; filtering the blend to recover the resulting solids; and dryingthe collected solids under vacuum to recover a powder having propertiesof both the polymer and preceramic metal oxide.
 2. The process of claim1 wherein the metal alkoxide solution is selected from the group ofmetal alkoxide solutions consisting of an alkyl titanate, an alkylzircononate and an alkyl silicate.
 3. The process of claim 1 wherein thepolar aprotic solvent is selected from the group of aprotic solventsconsisting of N-methyl pyrrolidinone (NMP), gamma-butyrolactone,N,N-dimethylacetamide, and 1,3-dimethyl-2-imidazolidinone.
 4. Theprocess of claim 1 wherein the step of refluxing the resultant solutionincludes providing a resin kettle equipped with a mechanical stirrer,nitrogen inlet, moisture trap and reflux condenser and containing aquantity of o-xylene reflux liquid.
 5. The process of claim 1 whereinthe polymer coating is a polyimide having repeating units of:

wherein Ar is an organic moiety selected from the group of organicmoieties consisting of:

wherein the catenation is meta, meta; meta, para; or para, para; whereinR is an organic moiety selected from the group of organic moietiesconsisting of:

and, wherein n is an integer in the range of 10 to 10,000.